Postretirement Benefits Other Than Pensions - Assumptions used to Determine Benefit Obligations (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Benefit Obligation
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year (as a percent)	6.43%	6.72%
Assumed decline in cost trend rate (as a percent)	4.50%	4.50%
Year of ultimate rate achievement	2038	2038
Weighted average discount rate (as a percent)	4.26%	3.53%
Net Periodic Benefit Cost
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year (as a percent)	6.72%	7.00%	7.27%
Assumed decline in cost trend rate (as a percent)	4.50%	4.50%	4.50%
Year of ultimate rate achievement	2038	2038	2038
Weighted average discount rate (as a percent)	3.53%	3.97%	4.27%
Effective rate for interest on benefit obligations (as a percent)	3.16%	3.32%	3.52%
Effective discount rate for service cost (as a percent)	3.77%	4.44%	4.68%
Effective rate for interest on service cost (as a percent)	3.59%	4.08%	4.37%